Technical Field
The present disclosure relates to transmission and reception of system information in a wireless communication system.
Description of the Related Art
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMES/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g., parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
The reception of system information (SI) is an operation to be performed by a UE on the basis of a scanned RF signal and a detected synchronization signal. In particular, upon the detection of synchronization signals the UE is capable of identifying a cell and of synchronizing with downlink transmissions by the cell. Accordingly, the UE may receive a broadcast channel, BCH, of a cell, and, hence, the corresponding system information. On the basis thereof, the UE can detect whether or not a cell is suitable for selection and/or reselection, i.e., whether the cell is a candidate cell.
System information is information which is transmitted in a broadcast manner to all UEs in a cell. It includes information necessary for cell selection and some parts thereof are to be read at any cell selection/reselection, after the UE synchronizes with the cell.
System information is structured by means of System Information Blocks (SIBs), each of which includes a set of parameters. In particular, system information is transmitted in a Master Information Block, MIB, and a number of System Information Blocks. The MIB includes a limited number of the most essential and most frequently transmitted parameters that are needed to acquire other information from the cell such as the downlink system bandwidth, an indicator of the resources allocated to HARQ acknowledgement signaling in the downlink, and the System Frame Number (SFN). The remaining SIBs are numbered; there are SIBs 1 to 13 defined in Release 8.
SIB1 contains parameters needed to determine if a cell is suitable for cell selection, as well as information about the time domain scheduling of the other SIBs. SIB2 includes common and shared channel information. SIBs 3 to 8 include parameters used to control intra-frequency, inter-frequency and inter-Radio Access Technology (RAT) cell reselection. SIB9 is used to signal the name of a Home eNodeB, whereas SIBs 10 to 12 include the Earthquake and Tsunami Warning Service (ETWS) notifications and Commercial Mobile Alert System (CMAS) warning messages. Finally, SIB 13 includes MBMS related control information.
The system information is transmitted by the RRC protocol in three types of messages: the MIB message, the SIB1 message and the SI message. The MIB messages are carried on the Physical Broadcast Channel (PBCH) whereas the remaining SIB1 and SI messages are at the physical layer multiplexed with unicast data transmitted on the Physical Downlink Shared Channel (PDSCH).
The MIB is transmitted at a fixed cycles. The SIB1 is also transmitted at the fixed cycles. In order to improve robustness of the system information transmission, the system information is repeated. The repetitions have different redundancy versions and thus, they are not repetitions of the bits effectively transmitted but rather repetitions of the data carried but coded differently. For instance, MIB is transmitted every frame in the first subframe (subframe #0) wherein the new MIB (MIB with content possibly different from the previous MIBs) is transmitted every four frames and the remaining three frames carry its repetition. Similarly, repetition coding is applied for transmission of SIB1. A new SIB1 is transmitted every 8 frames. Each SIB1 has three further repetitions.
All other SIBs are being transmitted at the cycles specified by SIB scheduling information elements in SIB1. In particular, the mapping of SIBs to a SI message is flexibly configurable by schedulingInfoList included in SIB1, with restrictions that each SIB is contained only in a single SI message, and at most once in that message. Only SIBs having the same scheduling requirement (periodicity) can be mapped to the same SI message; SIB2 is always mapped to the SI message that corresponds to the first entry in the list of SI messages in the schedulingInfoList. There may be multiple SI messages transmitted with the same periodicity.
Thus, a terminal determines the SI-window based on the signaled information and starts receiving (blind decoding) of the downlink shared channel using the SI-RNTI (an identifier meaning that signaling information is transmitted) from the start of the SI-window and continue for each subframe until the end of the SI-window or until the SI message was received, excluding the subframe #5 in radio frames for which SFN mod 2=0, any MBSFN subframes, and any uplink subframes in TDD. If the SI message was not received by the end of the SI-window, the reception is repeated at the next SI-window occasion for the concerned SI message.
In other words, during blind decoding, the UE tries to decode PDCCH on each subframe of an SI-window by using the SI-RNTI but only some of these subframes really carry PDCCH (CRC) encoded using the SI-RNTI (corresponding to PDSCH containing the particular SI).
For further details on the definition of system information, see for example 3GPP, TS 36.331, V12.5.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification (Release 12)”, sections 6.2.2.7 and 6.3.1, available at http://www.3gpp.org and incorporated herein by reference.
As LTE deployments evolve, operators strive to reduce the cost of overall network maintenance by minimizing the number of RATs. In this respect, Machine-Type Communications (MTC) devices is a market that is likely to continue expanding in the future.
Many MTC devices are targeting low-end (low cost, low data rate) applications that can be handled adequately by GSM/GPRS. Owing to the low cost of these devices and good coverage of GSM/GPRS, there is very little motivation for MTC device suppliers to use modules supporting the LTE radio interface.
As more and more MTC devices are deployed in the field, this naturally increases the reliance on GSM/GPRS networks. This will cost operators not only in terms of maintaining multiple RATs, but also prevent operators reaping the maximum benefit out of their spectrum (given the non-optimal spectrum efficiency of GSM/GPRS). With users and traffic becoming denser, using more spectral-efficient technologies, such as Long Term Evolution (LTE), allow the operators to utilize their spectrum in a much more efficient way.
Given the likely high number of MTC devices, the overall resource they will need for service provision may be correspondingly significant, and inefficiently assigned (for further details on objectives for MTC, see for example 3GPP, RP-150492 Ericsson: “Revised WI: Further LTE Physical Layer Enhancements for MTC”, section 4, available at http://www.3gpp.org and incorporated herein by reference).
Approaches to lower the cost of LTE presently regard the volume of products as the primary reason. The impact of volume can be seen in two possible ways, depending on how low-cost MTC is developed. Firstly, if low-cost MTC may be very similar to mainline LTE and included in LTE chipsets, MTC has the benefit of the volume of LTE. Secondly, a low-cost MTC based on LTE may have significantly lower cost than mainline LTE. Although it appears not to have the volume benefit of LTE, the volume of MTC devices can be even larger due to a potentially greater number of supported MTC applications and scenarios.
In this respect, the following approaches to lower the cost of LTE, i.e., defining low-cost MTC are discussed and found to have significant UE cost impact (for further details on low-cost MTC devices, see for example 3GPP, R1-112912, Huawei, HiSilicon, CMCC: “Overview on low-cost MTC UEs based on LTE”, section 4, available at http://www.3gpp.org and incorporated herein by reference):
Reduction in supported bandwidth for the low-cost LTE: The low cost of 1.4 MHz (6 RB) downlink bandwidth could cover most application scenarios of MTC. However, 3 MHz (15 RB) or 5 MHz (25 RB) could be considered given that the complexity does not increase much. Given that the uplink may have a larger requirement for MTC services, the possibility of reduced transmit power, and small baseband complexity (relative to downlink reception), any reduction in minimum transmission bandwidth in the UE should be carefully justified.
Modified PDCCH-related design for the low-cost LTE to simplify the PDCCH blind decoding and give efficient channel access for a large number of MTC devices. A reduction in maximum bandwidth (e.g., 1.4 MHz) decreases PDCCH blind decoding naturally.
Protocol simplification including HARQ consideration, MAC, RLC and RRC protocol. Signaling reduction between low duty cycle MTC devices and the base station.
Transmission modes down-selection to maintain coverage and balance complexity.
Further considerations on low-cost MTC devices relate to an improved indoor coverage. A number of applications require indoor deployment of Machine Type Communication, MTC, devices, e.g., in an apartment basement, or on indoor equipment that may be close to the ground floor etc. These UEs would experience significantly greater penetration losses on the radio interface than normal LTE devices. This effectively means that indoor coverage should be readily available and reliable: i.e., should provide a significant improvement on existing coverage.
Additionally, regarding the power consumption of low-cost MTC devices it is noted that many applications require devices to have up to ten years of battery life. In this respect, presently available Power Save Modes appear not sufficient to achieve the envisaged battery life. In this respect, it is anticipated that further techniques are proposed to significantly cut down the power usage of MTC devices e.g., by optimizing signaling exchanges in the system, in order to realize battery life of up to ten years.
For improving indoor coverage (for low-cost MTC devices), recent developments have focused on an Enhanced Coverage, EC, mode that is applicable to UEs e.g., operating delay tolerant MTC applications. Another term is “Coverage Extension”. The corresponding Work Item in 3GPP Release 12 “Low cost & enhanced coverage MTC UE for LTE” came to the conclusion that further complexity reduction of LTE devices for MTC can be achieved if additional complexity reduction techniques are supported, as apparent from the technical report TR 36.888, v12.0.0, “Machine-Type Communications (MTC) User Equipments (UEs)”, available at www.3gpp.org and incorporated herein by reference. The technical report TR 36.888 concluded that a coverage improvement target of 15-20 dB for both FDD and TDD in comparison to a normal LTE footprint could be achieved to support the use cases where MTC devices are deployed in challenging locations, e.g., deep inside buildings, and to compensate for gain loss caused by complexity-reduction techniques. MTC coverage enhancements are now expected to be introduced in 3GPP Release 13.
In general, the MTC devices may be low complexity (LC) MTC devices (which basically forces the device to receive a TBS of 1000 bits or less as a result of buffer size limitations and other implementation limitations) or enhanced coverage (EC) devices which are supposed to support a large number of repetitions.
In other words, LC are Low Complexity devices which are meant to be inexpensive devices with limited buffer sizes/simple implementation etc. whereas the EC devices are the coverage enhanced device that should operate in challenging situations like in basement or far away from the cell center.
The general objective is to specify a new UE for MTC operation in LTE that allows for enhanced coverage and lower power consumption. Some of the additional objectives are given below:
Reduced UE bandwidth of 1.4 MHz in downlink and uplink.
Bandwidth reduced UEs should be able to operate within any system bandwidth.
Frequency multiplexing of bandwidth reduced UEs and non-MTC UEs should be supported.
The UE only needs to support 1.4 MHz RF bandwidth in downlink and uplink.
The allowed re-tuning time supported by specification (e.g., ˜0 ms, 1 ms) should be determined by RAN4.
Reduced maximum transmit power.
The maximum transmit power of the new UE power class should be determined by RAN4 and should support an integrated PA implementation.
Reduced support for downlink transmission modes.
The following further UE processing relaxations can also be considered within this work item:
Reduced maximum transport block size for unicast and/or broadcast signaling.
Reduced support for simultaneous reception of multiple transmissions.
Relaxed transmit and/or receive EVM requirement including restricted modulation scheme. Reduced physical control channel processing (e.g., reduced number of blind decoding attempts).
Reduced physical data channel processing (e.g., relaxed downlink HARQ time line or reduced number of HARQ processes).
Reduced support for CQI/CSI reporting modes.
A relative LTE coverage improvement—corresponding to 15 dB for FDD—for the UE category/type defined above and other UEs operating delay-tolerant MTC applications with respect to their respective normal coverage shall be possible. At least some of the following techniques, which shall be applicable for both FDD and TDD, can be considered to achieve this:
Subframe bundling techniques with HARQ for physical data channels (e.g., PDSCH, PUSCH)
Elimination of use of control channels (e.g., PINCH, PDCCH)
Repetition techniques for control channels (e.g., PBCH, PRACH, (E)PDCCH)
Either elimination or repetition techniques (e.g., PBCH, PHICH, PUCCH)
Uplink PSD boosting with smaller granularity than 1 PRB
Resource allocation using EPDCCH with cross-subframe scheduling and repetition (EPDCCH-less operation can also be considered)
New physical channel formats with repetition for SIB/RAR/Paging
A new SIB for bandwidth reduced and/or coverage enhanced UEs
Increased reference symbol density and frequency hopping techniques
Relaxed “probability of missed detection” for PRACH and initial UE system acquisition time for PSS/SSS/PBCH/SIBs can be considered as long as the UE power consumption impact can be kept on a reasonable level.
Spreading: Spreading refers to spreading of information across resources including time-frequency domain resources or even spreading using Scrambling (or Channelization) codes.
There can be also other techniques than those listed above.
The amount of coverage enhancement should be configurable per cell and/or per UE and/or per channel and/or group of channels, such that different levels of coverage enhancements exist. The different levels of coverage enhancement could mean different level of CE techniques being applied to support the CE-device transmission and reception. Relevant UE measurements and reporting to support this functionality should be defined.
For more details, see for example 3GPP RP-141865 “Revised WI: Further LTE Physical Layer Enhancements for MTC” sourced by Ericsson, available at http://www.3gpp.org and incorporated herein by reference.
Notably, coverage enhancements of 15/20 dB for UEs in the Enhanced Coverage mode with respect to their nominal coverage means that the UEs have to be capable of receiving extremely low signal strengths. This applies not only to the initial scanning operation, the cell search and the cell selection operation but also the subsequent communication scheme to be performed by the UE. As described above, there will be different levels of CE depending on the network support and UE capability, e.g., 5/10/15 dB coverage extension.
Early attempts to define the Enhanced Coverage mode have focused on modifications of the radio transmissions. In this respect, discussions have focused on repeated transmissions as being the main technique to improve the coverage. Repetitions can be applied to every channel for coverage improvement.
An exemplary implementation of these repeated transmissions prescribes that the same data is transmitted across multiple sub-frames. Yet, it will become immediately apparent that these repeated transmissions will use more resources (time-frequency) than what is required for normal coverage UEs. RANI indicated that the transport block size used for transmission to the MTC devices will be less than 1000 bits.
The above requirements, a new information message scheduling will be necessary to minimize the system overheard as well as not to affect the system of previous releases and legacy UEs served thereby.